1. Field of the Invention
The present invention relates to a method for diagnosing deterioration of a catalyst used for purification of exhaust gas emitted from an internal combustion engine or for other purposes.
2. Related Background Art
Many catalysts for purification of exhaust gas have been developed so far for purification of harmful components emitted from internal combustion engines such as those of automobiles. For example, catalysts in which a noble metal such as platinum (Pt) and/or rhodium (Rh) is supported on a metal oxide support have been widely utilized as the catalysts for purification of exhaust gas. Moreover, catalysts have also been widely known in which a rare-earth metal oxide such as ceria or lanthana having an oxygen storage/release performance is used as the metal oxide in order to further improve the exhaust gas purification performance of the catalysts. Regarding such catalysts, for example, Japanese Unexamined Patent Application Publication No. 2009-84061 describes a catalyst for purification of exhaust gas in which a ceria-zirconia-based composite oxide is used as a catalyst having a crystal phase in which cerium ions and zirconium ions are regularly ordered (hereinafter, referred to as a regularly ordered phase).
As a method for diagnosing deterioration of such a catalyst for purification of exhaust gas, for example, Japanese Unexamined Patent Application Publication No. Hei 10-30999 describes a method for detecting deterioration of a catalyst, wherein a thermal history of a catalyst is detected by using a thermal history detection sensor comprising electrodes, a diffusion layer of a composite oxide having electrically insulating properties or the like, and an electrically conductive metal species such as a noble metal. However, such a thermal history detection sensor detects only aggregation of the noble metal, and cannot detect lowering in the oxygen storage/release performance.
Moreover, as a method for diagnosing deterioration of a catalyst by detecting lowering in the oxygen storage/release performance, for example, Japanese Unexamined Patent Application Publication No. 2009-191787 describes a method (so called the Cmax method) for diagnosing deterioration of a catalyst, wherein the air-fuel ratio of an exhaust gas flowing to a catalyst is forcibly switched to a rich side or a lean side, and an oxygen storage capacity of the catalyst is measured along with the switching, so that the deterioration of the catalyst is diagnosed. However, when such a Cmax method is carried out, it is necessary to dispose air-fuel ratio sensors both upstream and downstream of a flow path of the exhaust gas flowing to the catalyst. Moreover, in the Cmax method, the change in state of the catalyst is indirectly monitored by monitoring the air-fuel ratio in the exhaust gas flow path. Hence, the Cmax method has a problem that when the sensor itself is deteriorated, it is difficult to perform an accurate diagnosis. Note that, regarding such a sensor, for example, Japanese Unexamined Patent Application Publication No. 2007-315979 describes an oxygen sensor element in which Ce0.5Y0.5O2-δ is used as a temperature compensation material, and Ce0.9Zr0.1O2 is used as an oxide semiconductor, which is a gas detection material. However, this publication fails to describe deterioration of the sensor itself.
In addition, regarding the n-type semiconductor containing ceria, for example, Jean-Marie Herrmann, Catalysis Today, 2006, vol. 112, pp. 73 to 77 states that there is a correlation between an oxidation reaction rate of carbon monoxide using titania as a catalyst and the square of the electron conductivity of the titania. However, this publication is totally silent about the oxygen storage/release performance and the deterioration of the catalyst.